The cgs unit system

The cgs unit system is the abbreviation of centimetre-gram-second unit system. This unit system has three base units and these are the centimeter, the gram, and the second. The system was initially proposed in 1873 by the distinguished British scientists Lord Kelvin and James Clerk Maxwell, and the famous german electrical engineer Ernst Werner von Siemens. The unit system was oustanding for its consistency and for its clear distiction between force and mass. There are also advantages in the use of equations in four basic dimensions, one of which is electrical, and two fundamental sub-systems came into existence. The General Assembly of the IUPAP in Copenhangen, 1951, approved via its Resolution 5 the introduction of the following generalized cgs subsystems:

• the electrostatic cgs system (centimeter, gram, second, and franklin)
• the electromagnetic cgs system (centimeter, gram second, and biot)

The unit system was widely accepted by scientists in many countries and was rapidly extended to every branch of physics. However, many of its units are too small for most scientific and engineering purposes.

Officialy the cgs unit system was used unitl the introduction of SI unit system in 1960. Some practitioners in some fields of physics, such as electricity, magnetism, and optics, have continued to use unofficial derived units (e.g. dyne, erg, poise, stokes, gauss, oersted, maxwell, stilb, phot). The main reason for this is that these units are often of the same order of magnitude as the physical phenomena they define.
The major disadvantage of the cgs unit system is its inherent subdivision to three subsystems: electromagnetic units, electrostatic units, and the system of practical units for common use. The complications introduced by interconversion of these sub-units were yet another reason for its eventual abandonment in facour of the MKSA system and in the and the SI.

The esu subsystem

The esu is one of the cgs syubsystem in which the electrostatic force \(\mathbf{F}\) between two point charges \(q_1\) and \(q_2\) separated by a distance \(r\) in a medium of perimitivity \(\varepsilon\) is given by Coulomn's law $$ \mathbf{F} = \frac{q_1 q_2}{\varepsilon r^2} \mathbf{e}_r$$ if \(F, r, \varepsilon\) are maed equal to unity and \(q_1 = q_2 = q\), and \(q\) is unit electric charge. The cgs system of electrostatic units is based on this definition of electric charge. This unit is the franklin (Fr) which is the cgs unit of electric charge, and formally defined as:
The franklin is that charge whihc exerts on an equal charge at a distance of one centimetre in vacuum a force of one dyne.
All these units are prefixed with the separate acronym esu or an international or attached indicator stat. Example: statcoulomb or esu coulomb (=1 Fr).

The emu subsystem

The electromagnetic subsystem defines the electromagnetic force \(\mathbf{F}\) between two hypothetical isolated point magnetic poles of strenghts \(m_1\) and \(m_2\) separated by a distance \(r\) in a medium of magnetic permeability \(\mu\) by Coulomb's Law for magentism $$\mathbf{F} = \frac{m_1 m_2}{\mu r^2} \mathbf{e}_r $$ setting \(\mathbf{F}, r\), \(m\) equal to unity, and \(m_1 = m_2 = m\), \(m_1\) equal to unity , and \(m_1 = m_2 = m,\), \(m_1\) and \(m_2\) are unit pole strengths. the cgs system of electromagentic units is based on this definition of pole strenght, analogous with the electrostatic system. The cgs unit of magnetic pole strenght is called biot (Bi) and can be defined as:
The biot is constnat current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one centimetre apart in vacuo, would produce between these conductors a force equal to two dynes per centimetre of length.
All these units are prefixedd with the separate acronym emu, or attached indicator ab. Example: abampere or emuampere (=1 Bi)

Important information

The important notes about cgs unit system and esu and emu subsystems. The emu and esu are interconnected by the fundamental equation \(\varepsilon \mu c^2 = 1\) where \(c\) is the speed of light in vacuum. Thus the ratio of any pair of emu-esu primary units isequal to \(c\) or its reciprocal. $$ \frac{abampere}{statampere} = \frac{statvolt}{abvolt} = c $$ For esu or emu derived units, the ab/stat ratio is obtained by considering each of the primary units involved, thus: $$\frac{abfarad}{statfarad} = \frac{abcoulomb}{abvolt} = \frac{statvolt}{statcoulomb} = c^2$$ Since electromagnetic and electrostatic units vary enormously, a third cgs syubsystem was used for most practical purposes in electrical engineering. Crearly, however, this added considerable complication to its general structure. The use of the cgs system in fields other than mechanics involves exact definition of the subsystem concerned, which again addas to the confusion and is a great source of error in conversion computations.